Before taking off the useful layer, the donor wafer comprises a substrate and a useful layer that is to be taken off or transferred from the substrate. The useful layer is typically obtained by epitaxially depositing the layer on the substrate.
After removal, the useful layer is integrated with a structure in which components will be formed, particularly in the fields of microelectronics, optics, or optoelectronics, for the most part.
The layer to be taken off must therefore attain a high level of quality determined according to one or more specific criteria. The quality of the layer to be taken off largely depends on the growth support, that is, on the quality of the substrate on which it is epitaxially deposited.
The formation of such a high quality substrate is often complex and requires particular attention, involving technical difficulty and a raised economic cost. This latter point is further confirmed when considering the removal of a layer of a composite semiconductor material such as an alloy. In this situation, the epitaxy substrate also must exhibit a structure which is often difficult and costly to implement. Thus, substrates can be provided with a buffer layer to specifically avoid such difficulties of implementation.
The term “buffer layer” is generally understood to mean a transition layer between a first crystalline structure such as a support substrate and a second crystalline structure having as a first function a modification of the properties of the material, such as structural, stoichiometric properties or a surface atomic recombination. Buffer layers permit the support structure to include a second crystalline structure having a lattice parameter that differs substantially from that of the support substrate.
A first technique of forming a buffer layer consists in effecting the growth of successive layers so as to form a structure having a composition varying gradually in thickness, the gradual variation of components of the buffer layer then being directly associated with a gradual variation of its lattice parameter.
A layer, or superposed layers, formed on the buffer layer can be taken off from the donor wafer, and transferred to a receiving substrate so as to form a well-defined structure.
One of the main applications of a transfer of thin layers formed on a buffer layer concerns the formation of layers of elastically stressed silicon, and especially, in the case where the silicon is stressed in tension, because certain of its properties, such as electron mobility in the material, are then distinctly improved.
Other materials, such as for example SiGe, can also be the subject of a substantially analogous taking-off or transferring procedures.
A transfer of such layers onto a receiving substrate, specifically by a method termed SMART-CUT® that is known skilled artisans, then permits structures to be formed such as SeOI (Semiconductor On Insulator) structures.
For example, after taking-off an elastically relaxed layer of SiGe, the structure obtained, including the taken-off layer, can then serve as a growth support for silicon which will be placed under tension by the layer of relaxed SiGe.
As an illustration, an example of such a method is described in the IBM document of L. J. Huang et al., (“SiGe-On-Insulator prepared by wafer bonding and layer transfer for high-performance field-effect transistors”, Applied Physics Letters, 26, Feb. 2001, Vol. 78, No. 9) in which a process is given for forming an Si/SGOI structure.
Other applications of growth on a buffer layer are possible, particularly with Group III-V semiconductors. Transistors are thus commonly formed in technologies based on GaAs or based on InP. In terms of electronic performance, InP has an appreciable advantage over GaAs. For the main reasons of cost and feasibility, the chosen technique consists of transferring to a receiving substrate a taken-off layer of InP obtained by growth on a buffer layer on a support substrate of GaAs.
Certain taking-off methods, such as a method of the “etch-back” type, then entail a destruction of the remaining portion of the support substrate and of the buffer layer during taking-off. In certain other methods of taking-off, the support substrate is recycled but the buffer layer is lost.
The technique of formation of a buffer layer is complex. Moreover, to minimize its density of crystallographic defects, the thickness of a buffer layer is generally considerable, typically between one and several micrometers. The production of such a buffer layer thus leads to an often long, difficult and costly implementation.
A second technique of production of a buffer layer is disclosed in particular in WO 00/15885, which has as its main object to elastically relax a layer of Ge that is stressed by means of a Ge buffer layer.
This technique is based on specific epitaxy conditions, specifically associating the parameters of temperature, time, and chemical composition.
With respect to the first technique, it has the main advantage of being simpler, shorter, and less costly to perform.
The buffer layer finally obtained is moreover not as thick as a buffer layer formed according to the first technique.
A third technique of formation of a buffer layer is disclosed by B. Höllander et al., particularly in the document entitled “Strain relaxation of pseudomorphic Si1-xGex/Si(100) heterostructures after hydrogen or helium ion implantation for virtual substrate fabrication” (in Nuclear and Instruments and Methods in Physics Research B 175-177 (2001) 357-367).
It consists of relaxing elastic stresses present in the layer to be taken off by means of a deep hydrogen or helium implantation.
Thus from this point of view, this third technique can give a result close to a buffer layer produced according to one of the two previous techniques with substantially less demands of implementation.
The method describes specifically a relaxation of a SiGe layer stressed in compression, this layer being formed on an Si substrate.
The technique used comprises implantation of hydrogen or helium ions through the surface of the stressed layer in the Si substrate to a given depth, generating perturbations in the thickness of Si above the implanted zone (this thickness then forms a buffer layer) and causing, under heat treatment, a certain relaxation of the SiGe layer.
This technique seems to be shorter, easier to practice, and less costly than the first technique of forming a buffer layer.
An advantage of using this technique would be to later integrate this relaxed or pseudo-relaxed layer into a structure for the fabrication of components, particularly for electronics or opto-electronics.
However, in a manner similar to the first technique for forming a buffer layer, a buffer layer made according to one of the last two techniques is removed during the known techniques of recycling of the donor wafer after taking-off. Technical difficulties of implementation remain in carrying it out, so that improvement of the process is needed.